INTESTINAL CANAL OF THE ICHTHYOPSIDA. 387 



some such manner. I have met with individual Salamanders in 

 which the arteries in question were reduced in number. The 

 most striking example was one in which two, instead of four, 

 main trunks were represented (fig. 6, aim.) ; of these, the 

 anterior one occupied the position and had the relationships of the 

 anterior of the four more normally present, while the posterior 

 one coincided in origin with the normal second artery, but par- 

 took of the distribution of the posterior three usually present 

 {cf. fig. 5). These facts appear to me to indicate an origin of the 

 single artery of the higher forms by concrescence such as I have 

 postulated ; and, in further support of a belief in the same, 

 attention may be directed to the greater calibre of the posterior 

 of the two arteries (fig. 6) in the Salamander *, and to an indi- 

 vidual Frog (5, fig. 4), in which the single artery present 

 suddenly divided almost immediately after leaving the aorta 

 («o.). 



II. On the Arteries of the Coeliac and Superior Mesenteric 



Series, in the Ichthyopsida. 



I claim to have shown, in the foregoing, that those arteries 

 which in the Ichthyopsida supply the posterior segment of the 

 large intestine are serially homologous with the inferior mesen- 

 teric of the higher forms, and that the leading feature of these, 

 taken collectively, is the invariable disposition of their main 

 trunks at right angles to the axis of the body. In respect to 

 this they contrast most forcibly with the great arteries which 

 supply the rest of the alimentary canal ; those generally arise 

 far forwards, either from a single trunk (coeliaco-mesenteric) or 

 from two (coeliac and superior [anterior] mesenteric) or more 

 (cceliac, lieno-gastric, mesenteric, spermatico-mesenteric) trunks 

 well known (cf. Hyrtl 18, Parker 27, and fig. 6, i. to v.). 



The terminal portion of the intestine of the Plagiostome fishes 

 is well known (cf fig. 1) to be destitute of intestinal valve ; it 

 leaves the body in a straight line, becoming enlarged posteriorly 

 to form the cloaca (cV). To this valveless segment of the gut 

 Monro applied (33. p. 94) the term " great gut," but that of 

 " rectum " has been since more generally allotted it, apparently 

 on a supposed homology with the rectum of the higher Vertebrata 



* Wiedersheim figures an anastomosis between the third and fourth of the 

 series (33. p. 715, pi. 550 b). 



